A modern telemetry system comprises two major sections: a transmitter and a receiver. Concepts of data flow through these two sections are necessary to understand the invention as a useful element in a telemetry system.
The transmitter section of a telemetry system is concerned with the acquisition of raw telemetry data, conversion of its form and transmission of the converted data to the receiver section. A typical telemetry transmitter section includes an input analog-to-digital (A/D) converter, a commutator, a pulse code modulator (PCM) encoder and the transmitter itself.
While raw measurement data (from sensors, for example) is often available as an analog voltage or current, data is more conveniently and accurately transmitted in a digital, i.e., numerical, form. An A/D converter performs this task. The output of such a converter is a digital data word corresponding to the value of the input analog measurement data.
Since it is usually impractical to provide a dedicated telemetry data channel for each measurement, a commutator provides a means of time-sharing a single channel with data from multiple sources. A data commutator functions as a one-of-many switch; its output at any given time is a single selected input measurement. By regularly sampling measurement data in a specific sequence, the entire set of measurements can be assembled into a frame of data, containing a complete set of digital data words. Each frame also includes one or more special sync words, which are patterns for resynchronizing the data when it is recovered in the receiver section. A single stream of measurement data which issues from the commutator is a sequence of data words, with each word corresponding to a specific data measurement, plus appropriate sync words.
Precise and well-ordered as the data is at this point, it is not ready for transmission over a single data channel. Word-sized, parallel digital data must first be converted into a single serial bit stream. Although it is possible to transmit a raw serial bit stream directly, other types of PCM data encoding are often desirable, i.e., to enhance recording, or error detection/correction. Specifically, at least ten different PCM codes find common use in telemetry today. A PCM encoder has the task of converting the serial bit stream into an appropriate PCM bit stream.
An actual telemetry transmitter typically comprises a modulator, a radio frequency (RF) transmitter and an antenna. The output PCM bit stream from the PCM encoder is used to modulate the transmitted RF signal. Specific modulation techniques vary according to equipment and application. Since a telemetry transmitter function need not be wireless, modern data conversion techniques apply equally well to wire transmission.
The telemetry receiver section performs an inverse function to the transmitter. Data is received, converted in form, and reconstituted. A telemetry receiver section, typically a telemetry ground station, includes a receiver, a PCM bit synchronizer/decoder, and a decommutator. The telemetry receiver is directly compatible with the telemetry transmitter already described. For a typical RF data link, it includes an antenna, a receiver, and a deomodulator. The output of the telemetry receiver is a reconstructed version of the serial PCM data which modulated the transmitter.
Reconstructing the numerical form of data begins with the PCM bit synchronizer/decoder, which has the task of regenerating the original serial bit stream. The PCM decoding process entails regeneration of bit-level timing data, i.e., bit synchronization, as well as data decoding. The output of the bit synchronizer/decoder is a serial bit stream and bit-rate clock (timing) signals.
The decommutator has the task of reconstructing the serial bit stream back into data words and frames of data words. By recognizing the special frame synchronization pattern data, the decommutator can reestablish timing for the serial bit stream of both the word and frame levels. Given a known word size and frame structure, it is possible to reconstruct the data first as words, (serial to parallel data conversion) and then as frames of words. Data output from the decommutator is in the form of a stream of parallel data words, with accompanying word and frame clock (timing) signals. This form of data is convenient for recording, display, and "real-time" data processing.
Multiple levels of commutation are sometimes used in telemetry. A common technique is to define minor frames comprised of data words and major frames comprised of minor frames. The process of decommutation of such data is similar; additional synchronization information is added to the transmitted data stream to allow the decommutator to identify both major and minor frame boundaries.
Recorded decommutator output data are usually played back for comprehensive analysis, which usually requires consideration of specific timing information which is typically not available in the data itself. Consequently, it is often desirable to time-tag recorded frames of data. A convenient source of millisecond-resolution time data for this purpose can be provided by a serial time coder reader. Synchronous time data can be generated at a single source and distributed to multiple destinations in this manner. Two examples of common serial time codes are IRIG-B and NASA-36.
A time coder reader accepts serial time code input and converts it to a usable numerical form for general use, including real-time display and time-tagging of recorded data. In the absence of input serial time code, the same time code reader typically provides a "generator" mode, which can generate its own output time data. The output time data is typically added to the beginning or end of each frame of telemetry data as it is recorded.
An analog representation of telemetry data, output to a chart recorder or oscillograph, can be useful, especially in real-time operation. As a result, most telemetry ground stations provide a multi-channel digital-to-analog (D/A) converter specifically for this purpose. In operation, a D/A converter must periodically sample specific output words from the decommutator's output stream of data words. The sampling process must be properly timed with respect to the decommutator, and special techniques may be required to extract specific data fields, or portions thereof, of output data words for analog conversion and output. The analog output form is typically multiple channels of voltage, suitable for driving analog display or recording devices.
The need for a compact, portable, inexpensive, computer-based PCM telemetry system has existed for some time. Prior art PCM telemetry systems consist of discrete pieces of specialized electronic equipment, such as a PCM bit synchronizer, a PCM decoder, a time code reader, a decommutator, a computer, a cathode ray tube (CRT) terminal (keyboard and display), digital disk and tape recorders, D/A converters and the like. Such prior art telemetry systems typically require a mainframe computer, which is in itself expensive, and associated floor space, air conditioning and electrical power. Furthermore, experienced software programmers are required to write custom programs to reduce data on such telemetry systems. In such prior art systems, each new or changed data reduction requirement must be answered with modifications to the custom software.
In response to the clear needs for a less expensive and truly portable, real-time PCM telemetry system, the presence invention provides a solution based upon an IBM model PC-AT or compatible commercially available desktop microcomputer. Such microcomputers are readily available in the market, are inexpensive, simple to service and require no special environment as do larger mainframe computers. The IBM PC-AT microcomputer's industry standard Intel 80286 microprocessor chip can function as a central processing unit (CPU), since it has adequate computing power and a large, mature base of useful software. Those skilled in the art will appreciate that faster versions of a microcomputer with 8, 10 or 12 megahertz or higher clock rates could also be utilized to practice the invention. Similarly, faster compatible microcomputers employing the Intel 80386 microprocessor chip, which is fully compatible with the 80286 microprocessor, can be utilized to practice the invention. Further, the hardware and software utilized in practicing the invention are general enough to be applied to other type of microcomputers which utilize different types of microprocessors. However, for purposes of presenting a preferred embodiment, the conventional 8 megahertz IBM PC-AT or compatible microcomputer and parameters related thereto are discussed herein.
The invention provides three major improvements over prior art telemetry systems: integration of specialized telemetry and computer hardware; integration of hardware and computer software; and built-in generic software for telemetry data collection and reduction. The integration of specialized telemetry and computer hardware is complete to the extent of providing a workable integrated PCM ground station with the following equipment: a PCM bit synchronizer/decoder, a time code reader, a decommutator, a computer, a CRT display, a keyboard, digital disk and tape recorders, D/A converters and a hardcopy printer. The invention provides integrated software to configure and control all its specialized telemetry hardware. Hardware and software are dependent on each other to achieve the tasks of collecting, converting, recording, displaying and reducing telemetry data. Built-in generic software supports telemetry data reduction in practicing the invention, so that the writing of custom software programs in the classical sense is not necessary. Instead, an interactive control program allows the user to configure the complete system and data reduction procedure by selecting operating parameters, names and definitions of specific parameters, and general computing algorithms to be used in reducing data. Entire sets of configurations data for the system can be easily stored, recalled, edited, and utilized.
Starting with a standard IBM PC-AT microcomputer or compatible commercially available device, specialized telemetry hardware is added internally on three novel printed-circuit cards, which contain a PCM bit synchronizer/decoder, a time code reader, a decommutator, and a 16-channel D/A converter. Novel architectures for electronic control circuitry which incorporate programmable state machines are used in the PCM bit synchronizer/decoder, time code reader, and D/A converter functions to minimize random logic circuit requirements. The bit synchronizer circuit also features a novel completely digital phase-locked loop circuit architecture to replace classical (analog) equivalent circuitry.
The invention also uses software and hardware combinations in lieu of certain devices which have been implemented in pure hardware in prior art telemetry applications. For example, the conversion of time code reader output time data in binary coded decimal (BCD) form to packed binary form (with status) in practicing the invention depends both on the time code reader hardware and special time code reader support software; hardware and software perform interdependently to achieve the required function. Likewise, the high-speed transfer of telemetry data to the mass storage disk in the invention is achieved using both hardware and software in an interdependent manner.
An attractive feature of the invention is that it also augments the normal mass storage capabilities of the microcomputer by adding internally a 51/4 inch hard disk of 120 megabyte or greater capacity and externally a 10-inch, 9-track digital tape drive, both of which are commercially available items. A commercially available switched power distributor unit is fitted with a special telemetry indicator panel on its front and a telemetry signal patch panel on its rear for application as part of the invention.